Add-Ons are additional items that are sold along with a listing. They are set on a per-listing basis and you can add as many as you like. Add-ons can be customer-facing (so available to purchase during online checkout) or private (so can only be added to a purchase if it is a back office booking or modification).
Structure
The add-on structure is very simple. It has a name, a price, a description and a visibility flag.
{
"name": "Go-Pro",
"price": 15,
"desc": "A Go-Pro camera to record your trip!",
"visibility": "public"
}